In Lesson 2A of the Genesis study, Pastor Steve mentions that the Garden of Eden stood for 700 years after Adam's death. Where does he get this figure?
We begin to get these dates from Genesis 5. We know that Adam died when he was 930 (v.5) and by adding the dates in which each generation was born we can determine that Noah was born 126 years after Adam died, making his birth 1056 years after creation. In Genesis 7:6 we find out that Noah was 600 years old when the flood came upon the earth. Therefore we know that the flood happened 1656 years after creation and therefore 726 years after Adam died.
When Pastor Armstrong said "700 years after Adam's death" he was using approximation rather precision in that moment.
We believe that the Garden of Eden stood until the flood because at the time of Adam and Eve's exile the Lord didn't destroy it. He stationed cherubim to guard it, says Genesis 3:24.
In the absence of mention of any other destructive forces at that time, we infer that the Garden simply stood in place until the flood came up and destroyed every living thing, and catastrophically changed the earth's landscape (see Genesis 7:17-24).